The present invention is related to optical photolithography, and more particularly to the measurement of overlay errors and optical aberrations.
The manufacture and fabrication of semiconductor devices involve complex processing steps. During the manufacture of integrated circuits, many layers of different materials are applied to a substrate. These layers overlie one another and must be accurately registered to ensure proper operation of the semiconductor device. If the layers are not properly aligned, the device may not perform well, or may even be inoperative. As semiconductor devices have increased in complexity, the feature dimensions of these devices have decreased, and the influences of optical aberrations become more significant.
To aid in the registration of overlying layers in semiconductor devices, registration patterns, or marks, are included in each layer of the wafer used during fabrication. These patterns have a predetermined relationship when they are correctly registered. A reticle is used to pattern the appropriate marks on a particular wafer process layer, such that the marks can be readily identified by a Registration tool in subsequent processing steps. One example of an alignment mark is a box-in-box mark. An outer box is formed by photolithography, and an inner smaller box is formed in a separate photolithography layering step. When the two boxes are concentric, the layers are accurately registered. Any alignment error produces a displacement of the boxes relative to each other.
Because semiconductor devices are complex and expensive to fabricate, it is desirable to verify registration after the application of each layer. If the displacement of layers is outside of the acceptable limits, defective layers can then be removed and replaced. Registration measurement, verification, and correction is therefore critical to the successful fabrication of these semiconductor devices.
Registration measurement, verification, and correction can be limited by optical aberrations introduced during the photolithography process. Aberration errors are of particular significance given the reduction of sizes of patterns in semiconductor devices. Aberrations affect the ability to accurately measure overlay error. Shift quantity measurements may not correspond to the actual shift quantities.
There are different forms of aberrations that can affect registration verification. Coma aberration exerts the largest influence on the determination of overlay error. Shift of a wave front caused by coma aberration is large at a peripheral portion of a lens and is small at a central portion. Diffracted rays of a large semiconductor pattern are pot significantly affected by coma aberration because they have a small diffraction angle and pass through a central region of a lens, causing less wave front aberration. However, a small semiconductor pattern allows passage higher frequency light, which will be more affected by a diffraction phenomenon of a lens. Therefore, the rays diffracted by a small semiconductor pattern have a large diffraction angle, and pass through a peripheral region of a lens, thereby exhibiting more of a coma aberration.
Astigmatism is another optical aberration that occurs because a wave surface in general has double curvature. In this form of aberration, the rays from an object point do not come to a point focus, but rather intersect a set of image planes in a set of ellipses, the diameters of which are proportional to the distances of the two foci from the image plane in consideration.
Spherical aberrations have symmetry of rotation, and are direction-independent. These aberrations occur because rays of different aperture usually do not come to the same focus. These aberrations are also sometimes referred to as aperture aberrations. Spherical aberration occurs in simple refraction at a spherical surface, and is characterized by peripheral and paraxial rays focusing at different points along the axis.
As discussed earlier, semiconductor devices have increased in complexity. The feature dimensions of these devices have decreased, and the influences of overlay errors and optical aberrations have become more significant. It is critical that both overlay errors and optical aberrations be estimated accurately and easily to optimize the critical dimension manufacturing process.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need for an alignment mark that can be used to estimate both overlay errors and also optical aberrations such as astigmatism, coma, spherical aberration, and defocus.
One aspect of the present invention provides an aberration mark for use in an optical photolithography system, and a method for estimating overlay errors and optical aberrations. The aberration mark includes an inner polygon pattern and an outer polygon pattern, wherein each of the inner and outer polygon patterns include a center, and two sets of lines and spaces having a different feature size and pitch that surround the outer polygon pattern. The aberration mark can be used to estimate overlay errors and optical aberrations.
In some embodiments, the inner polygon pattern is a smaller square box shape and the outer polygon pattern is a larger square box shape. In other embodiments, the inner polygon pattern is a smaller octagon shape and the outer polygon pattern is a larger octagon shape.
Another aspect of the present invention provides a method of using an aberration mark during a scatterometry process to estimate optical aberrations, wherein the aberration mark has two schnitzel patterns of different pitch. The method includes shining a laser on the aberration mark at an angle, capturing an image of a scattering of the laser from the two schnitzel patterns, measuring a width of the two schnitzel patterns, estimating a defocus aberration, estimating a coma aberration, estimating a spherical aberration, and estimating an astigmatism aberration.
Yet another aspect of the present invention provides a method of using a mark with a scanning electron microscope to estimate overlay errors and optical aberrations, wherein the mark has a box-in-box structure and two schnitzel patterns of different pitch that surround the box-in-box structure. The method includes scanning the mark with an electron beam in a vacuum, capturing an image of ejected electrons from the two schnitzel patterns, measuring a width of the two schnitzel patterns, estimating a displacement of the box-in-box structure, estimating a defocus aberration, estimating a coma aberration, estimating a spherical aberration, and estimating an astigmatism aberration.
Still another aspect of the present invention provides a method for monitoring aberrations of a lens in an optical photolithography system. The method includes forming a reticle on a first mask, the reticle having a box-in-box structure and two schnitzel patterns of different pitch that surround the box-in-box structure, forming a first image pattern from the reticle during a first photolithography cycle, the first image pattern having a box-in-box structure and two schnitzel patterns of different pitch that surround the box-in-box structure of the first image pattern, measuring a first line-shortening effect in the two schnitzel patterns of the first image pattern, estimating a baseline set of optical-aberration values of the lens, forming the reticle on a second mask, forming a second image pattern from the reticle during a second photolithography cycle, the second image pattern having a box-in-box structure and two schnitzel patterns of different pitch that surround the box-in-box structure of the second image pattern, measuring a second line-shortening effect in the two schnitzel patterns of the second image pattern, estimating a subsequent set of optical-aberration values of the lens, and comparing the baseline and subsequent set of optical-aberration values of the lens to determine changes.